1. Field of the Invention
The present invention relates to curable silicone compositions. More specifically, the present invention relates to curable silicone compositions having an improved depth of cure and which incorporate a fluorescent agent detection system.
2. Brief Description of Related Technology
A variety of silicone compositions have been developed that may be used as sealants, gaskets, conformal coatings, potting compounds, encapsulants, and the like. Among the numerous silicone compositions that have been created, some depend on atmospheric humidity and/or moisture to be present on the substrate to which the composition is applied, for their cure. Although such moisture-curing silicone systems can provide good physical properties and performance when fully cured, they suffer the disadvantage that the moisture curing process is relatively slow.
In consequence, silicone compositions that depend on other curing processes have been developed which are significantly faster than those that undergo a moisture-curing process. In particular, photo-curable (e.g. UV-curable) silicones have been developed which offer a fast curing speed, far superior to conventional moisture-curing silicones. These silicone compositions may include a photoinitiator that provides a faster and more effective cure. Silicone compositions that are both moisture-curable and photo-curable have also been developed that provide a rapid cure. Examples of these are described in several patents assigned to Henkel Corporation, including U.S. Pat. Nos. 5,663,269, 5,516,812, 5,498,642 and 5,348,986 to Chu et al.; 6,451,870 to DeCato et al; 6,140,444 to Levandoski et al.; 4,528,081 to Lien et al.; and 4,699,802 to Nakos et al., all of which are hereby expressly incorporated by reference.
In addition to cure speed, cure-through-depth (CTD), also sometimes referred to as cure-through-volume (CTV), is an important feature to many applications. Conventional photo- and/or dual curing silicones have shown only limited depth of cure. Many silicone compositions, and particularly photo-curable silicone compositions, are unable to fully cure at depths greater than 5-8 mm. Many applications, such as potting and gap filling of electronic components require compositions which can effectively cure through relatively large depths or volumes.
While a variety of different photoinitiators are generally known as being useful for curing silicones, only a limited number are known to have produced a substantial increase in CTD. One such case is disclosed in U.S. Pat. No. 6,627,672 to Lin et al., which discloses a photo-curable silicone composition containing a premix which includes an acylphosphine oxide and at least one polar carrier. This patent also discloses a dual photo/moisture curing silicone composition which includes a photo/moisture curable silicone, a premix including a first photoinitiator comprising an acylphosphine oxide, a second photoinitiator different than the first photoinitiator and at least one polar solvent.
Polymerizable silicone compositions are frequently clear or transparent when cured. As a result, the cured compositions may be difficult to see. Thus, the presence of the silicone, or the quality of the application coverage is therefore hard to access and/or correct.
Various agents have been incorporated into polymer systems in general for detection purposes. Fluorescing agents have been incorporated into curable compositions to provide a non-destructive method of inspection and identification of cured films, and ensuring proper coating of the composition on an article. For example, U.S. Pat. No. 6,080,450 to Cantor discloses a polymerizable acrylate formulation which contains a fluorescing agent. Dyes and pigments are also known agents which are used to provide visible identification means to curable resins. Such agents, however, are not necessarily useful in photo-curable resins because they impair the ability of light to initiate polymerization. Moreover, fluorescing agents are known to reduce the depth of cure in photo-cure compositions since they absorb the light required for polymerization. As a result, photo-curable silicone resins have not included fluorescent agents. Additionally, high-depth photo-curing silicones would be expected to be particularly hampered by the incorporation of a fluorescent agent.
Accordingly, there is a need for a photo-curable silicone having improved CTD and which permits detection of the cured polymer through the incorporation of a fluorescent agent.